The invention relates to the field of digital radiology image systems, and in particular to a real time method and apparatus for determining the appropriate window and level settings used while displaying the digital images.
One of the handicaps of current digital radiology systems is that display technology does not measure up to the dynamic range capabilities of radiographic acquisition systems. As a result, currently available display terminals can only deliver part of the dynamic range present in Picture Archiving and Communications System (PACS) images commonly used in the medical field. Images such as X-rays can be acquired, stored and fed to a radiographic workstation at 12 or 14 bits per pixel. (Pixels values range from 0 to 4095 (4096 values) for 12-bit pixels.) Display equipment, however, typically supports only 8 bits per pixel. (Pixel values range from 0 to 255 (256 values) for 8-bit pixels.) Therefore, acquired images represented by pixels capable of having 4096 different values are displayed on monitors that can only display pixels with 256 values. This loss in dynamic range requires enhancement of the displayed image by the workstation operator (e.g., the radiologist).
Some images, for example, computed tomography (CT) images, do not undergo such a drastic loss in dynamic range. Nevertheless, these images still require the enhancement of particular (task-specific) segments of the dynamic range to bring (task-dependent) features of interest in focus. Since most images will be multimodal (that is, they contain more than one structure of potential interest, for example, bone or tissue) there exists a need to look at specific structures within the image. Again, this enhancement would be performed by the radiologist.
In practice, the radiologist enhances the displayed image by controlling the effective dynamic range of the image. This enhancement is done by setting two display parameters known as the "window" and "level" settings. The radiologist looks at the dynamic range of the image through a window of a specified width which is centered at level (see FIG. 2).
The system shown in U.S. Pat. No. 5,447,153 is typical of an apparatus that requires the operator to manually select window and level settings before any enhancement is made to the image. An interpretation session is used by the radiologist to view images of the patient and to determine the extent of the patient's ailment. In a typical interpretation session, the radiologist will first attempt to find a reasonably good approximation for the window and level settings to bring the study into focus (that is, to make all of the structures of interest visible). This initial step takes valuable time and effort even for experienced radiologists. An experienced radiologist may spend 20 to 40 percent of the interpretation session just to get the image in focus. Inexperienced radiologists may never obtain suitable window and level settings, resulting in an unreadable image, which may lead to an improper diagnosis. Difficulty in obtaining the correct window and level settings are attributable, in part, to the fact that the displayed image initially comes up in an almost unreadable format.
Prior attempts have been made to automate this procedure. For example, U.S. Pat. No. 5,542,003 refers to a method for maximizing the dynamic range for a region of interest within a medical image. The disclosed method utilizes a simplistic algorithm that operates on an image histogram of the region of interest selected by the operator, but sets window and level parameters based on minimum and maximum pixel values within the region of interest (ROI). In addition, the algorithm rejects a predetermined percentage of pixel values at the endpoints of the histogram.
U.S. Pat. No. 5,305,204 refers to an apparatus with automatic window and level adjustment. The disclosed apparatus utilizes an algorithm that operates on an image histogram of the region of interest selected by the operator, but sets window and level parameters by compensating for a dark background, calculating initial settings, assessing image quality, and modifying the settings. The procedure is repeated until a proper window and level setting is acquired.
These prior attempts to automate the window and level adjustments, however, can not furnish multiple window and level settings required to properly visualize and interpret multimodal images. As such, these prior art attempts can not provide a menu of optimal window and level settings corresponding to the different types of information the radiologist may be interested in (for example, blood vessels, lung nodules, soft tissue, microcalcifications, etc.). In addition, these prior art attempts do not always display an initial image that is presented in a readable format. Without these features, the interpretation of the patient's image will be difficult and may lead to an improper diagnosis.
A window and level control tool that operates in real time and can quickly determine initial values for window and level settings such that the displayed image is always presented in a readable format is desired to solve the aforementioned problems. It is desirable that any such tool provide a menu of optimal window and level settings corresponding to the different types of information the radiologist may be interested in (for example, blood vessels, lung nodules, soft tissue, microcalcifications, etc.). The number of menus should be limited to optimal settings, not an endless list of "equally good" alternatives. It is desirable that any such tool relies on image statistics in addition to suspected pathology. It is desirable that any such tool be "generic" in that it does not need to be tied to any particular medical imaging modality, or the type of study being conducted. In addition, it is desirable that any such tool be customizable to fit the needs of individual radiologists. Prior art systems or tools have not been able to accommodate these needs.